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FIG. 6.2.2 Range of industrial plant noise levels at operator's position. (Reprinted from U.S. Environmental Protection Agency)

plicated. In nonrotating machines (transformers, magnetic relays, and switches), the noise frequencies are the line frequency and its harmonics and the frequencies of vibration of small parts which are driven into vibration when their resonant frequencies are near some driving frequency.

In many machines, more noise is produced by the material being handled than by the machine. In metal-cutting or grinding operations, much noise is produced by the cutting or abrading process and is radiated from both work-piece and machine.

Belt and screw conveyors are sometimes serious noise sources; they are large-area sources; their own parts vibrate and cause noise in operation, and the material they handle produces noise when it is stirred, dropped, or scraped along its path of motion. Vibration from conveyors is conducted into supports and building structure as well. Feeding devices, as for automatic screw machines, often rattle loudly.

Jiggers, shakers, screens, and other vibrating devices produce little audible noise in themselves (partly because their operating frequency is so low), but the material they handle produces much higher frequency noise. Ball mills, tumblers, and the like produce noise from the many impacts of shaken or lifted-and-dropped pieces; their noise frequencies are often low, and much mechanical vibration is around them.

Industry uses many pneumatic tools. Some air motors are quite noisy, others less so. Exhausting air is a major noisemaker, and the manner in which it is handled has much to do with the noise produced. Exhausting or venting any gas (in fact, any process which involves high velocity and pressure changes) usually produces turbulence and noise. In liquids, turbulent flow is noisy because of cavitation. Turbulence noise in gas is usually predominantly high frequency; cavitation noise in liquids is normally midrange to low frequency. Both types of noise can span several octaves in frequency range.

Gas and steam turbines produce high-frequency exhaust noise; steam turbines (for improved efficiency) usually exhaust their steam into a condenser; gas turbines sometimes feed their exhausts to mufflers. If such turbines are not enclosed, they can be extremely noisy; turbojet airplane engines are an example.

Impact noises in industry are produced by many processes; materials handling, metal piercing, metal forming, and metal fabrication are perhaps most important. Such noises vary widely because of machine design and location, energy involved in the operation, and particularly because of the rate of exchange of energy.

Not all industrial noises are within buildings; cooling towers, large fans or blowers, transformer substations, external ducts and conveyor housings, materials handling and loading, and the like are outside sources of noise. They often involve a large area and contribute to community noise. Bucket unloaders, discharge chutes, and carshakers, such as those used for unloading ore, coal, and gravel, pro duce noise which is more annoying because of its lack of uniformity.

Figure 6.2.2 summarizes a range of industrial plant noise levels at the operator's position. Table 6.2.3 gives some industrial equipment noises sources.

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